Laminated glass assemblies have been prepared by bonding two plate glass templates together with an intermediate plastic layer such as polyvinyl butyral layer. These laminates have been widely used as windshields for automobiles. The two glass templates have been made thinner over the years to help reduce the weight of automobiles. In order to improve their strength, the glass is generally subjected to annealing or tempering. This thinner glass may be chipped or broken when a shock is delivered to the edges of the glass, which may take place during transportation of the glass or when the assembly is being embedded in the frame of an automobile.
Methods have been proposed to improve the edge strength of plate glass (float or sheet) and hence reduce its fragility. U.S. Pat. No. 3,843,472 to Toussaint et al. discloses improving the edge strength of a glass article by abrading the edge of the glass to smoothen or polish it and subjecting it to chemical tempering and a rectifying treatment. Abrading or grinding the edges of glass, also commonly termed "chamfering" or "seaming", is routinely used in the glass manufacturing industry to improve the edge strength of glass. U.S. Pat. No. 4,075,381 to Furukawa et al. discloses a method to improve the edge strength of a laminated windshield by providing the laminated windshield with a particular compressive stress in the outer surfaces.
In forming a glass windshield two plate glass templates 12 and 14 are positioned relative one another as shown in FIG. 1, and would include therebetween in the finished product an interposed plastic layer 16 to form a laminated windshield. Plate glass template 12 has two major surfaces, conveniently spoken of as surfaces #1 and #2 in the glass industry. The #1 surface is the smooth major surface and faces outside toward the environment when the windshield is installed in a vehicle. It is considered in the glass industry to be the "smooth" major surface because it is the surface in direct contact with the molten tin during the float-bath production of the plate glass. Float-glass processing is the conventional way of producing sheet glass, used for automotive and architectural uses, throughout the world. Major surface #2 is considered the rough major surface, i.e., compared to major surface #1, simply because it was not formed in direct contact with the molten tin bath. Plate glass template 14 of FIG. 1 likewise has a smooth major surface, surface #4, and a rough major surface, #3. Surface #4 would face the inside of the vehicle when installed. Therefore, major surfaces #2 and #3 are in facing retention in a windshield assembly.
In current conventional production of a laminated windshield, a first plate glass template, e.g., 12' of FIG. 2, is formed by scribing an indentation into the rough major surface of a glass sheet in a desired window pattern and then the glass outside the scribed pattern is removed. This cut edge (i.e., the edge adjacent major surface #2 of template 12' of FIG. 2) is chamfered as shown as 22 in FIG. 2. The so-called "uncut" or "broken" edge (i.e., the edge adjacent major surface #1 of template 12' of FIG. 2) is likewise chamfered as shown as 20 in FIG. 2. Plate glass template 14' of FIG. 2 (the second template in the windshield) is produced differently from template 12' in that, in current conventional production, it is smooth major surface #4 which is scribed. The cut edge is chamfered as shown as 26 in FIG. 2 and likewise the uncut edge is chamfered as shown as 24 in FIG. 2. As discussed above, rough major surfaces #2 and #3 are in facing retention, with surface #1 facing the outside of the vehicle and surface #4 facing the inside of the vehicle when installed. It was previously theorized that this is the optimal way to manufacture the glass template pairs having the greatest edge strength because any micro-cracks which may have been formed in cut edges #2 and #4 would be "sealed up" as the windshield is curved back toward surface #4 during windshield shaping.
It would be highly desirable if the edge strength of the plate glass template pairs and the resulting laminated windshield could be further improved. This would reduce even further the number of template pairs and windshields which are scrapped because of breakage during storage, shipment, or attempted installation into a vehicle. It would be further desirable if this could be done by a less costly and time consuming method than that currently employed in manufacturing windshields. The present invention has unexpectedly been found to provide significant edge strength improvements over those conventionally carried out as described above and does so by a less complex method.